1. Field of the Invention
This invention relates generally to the field of microphones, and in particular, to a system and method for a monolithic directional microphone array.
2. Background Art
There are two general types of prior art microphones. The first type is the stand-alone microphone. Stand-alone microphones suffer from a number of disadvantages. First, these microphones cannot differentiate between two or more acoustic signals having different frequencies or originating from different spatial locations. Second, these microphones are unable to adapt to changing sources of sound, and are unable to track a moving source of sound.
The second type of prior art microphones is actually a microphone system which includes signal processing capabilities that can track and adapt to changing sources of sound. Unfortunately, these microphone systems are expensive, bulky and not suited for home use.
Noise cancelling microphones represent one type of prior art system that can track and adapt to changing sources of sound, and are commonly employed, for example, in helicopters. Such a noise cancelling microphone includes one microphone to record the speaker's voice, a second microphone to record the background noise, and a noise reduction circuit that subtracts the background noise from the speaker's voice to improve the signal quality of the speaker's voice. Although the noise cancelling microphone is suitable for noisy environments, these microphones suffer from several disadvantages. First, noise cancelling microphones cannot track a moving sound source, nor can they selectively adapt to a particular spatial angle. Second, they are costly.
Another example of prior art systems that can track and adapt to changing sources of sound are those employed by the military. Military directional acoustic detection systems are adept at tracking a changing sound source. These systems employ digital signal processing (DSP) techniques such as adaptive beam forming and noise reduction (commonly referred to as null steering) to improve signal quality. These systems, such as sonar systems, are commonly employed in submarines and ships. However, these prior art directional systems suffer from the drawbacks that they operate in a water medium and are bulky in nature. For example, the transducers employed in a towed array or mounted on the hull of a ship are large, heavy and unwieldy to maneuver. Moreover, the signal processing units are complex and often occupy several rooms of space.
Yet another example of prior art systems that can track and adapt to changing sources of sound are the ADAP 256 and ADAP 1024 systems that were sold by the assignee of the present application. These systems were used by law enforcement agencies, and are capable of performing functions such as frequency discrimination, separating the speakers' voices (i.e., sounds) based on correlation times, and removing background sounds. However, these systems are bulky (about 19 inches wide by 24 inches deep by 5 inches high) and expensive.
Accordingly, the size, complexity, and cost of the transducers and signal processing units required by the prior art systems that are capable of tracking and adapting to changing sources of sound hinder the use of these systems in consumer household electronics.
Accordingly, there remains a need for a system and method for a monolithic directional microphone array that can track and/or locate a changing source of acoustic waves or noise, that can separate components of a sound field, selectively enhance each component and selectively recombine them, and that is compact, portable, and cost effective.